This invention relates to a process for electroless copper plating and an apparatus used therefor. More particularly, this invention relates to a process for electroless copper plating suitable for producing printed wiring boards having fine circuits by an additive method, and an apparatus used therefor.
It has been known in carrying out electroless copper plating to introduce an oxygen-containing gas such as air into a plating solution in order to stabilize the plating solution. That is, in the electroless plating solution, there is a tendency to lower a dissolved oxygen concentration by the generation of hydrogen gas during the plating reaction, and also the dissolved oxygen seems to be consumed by an oxidation reaction of Cu(I) which seems to be a by-product as follows: ##STR1## In order to compensate the lowering in the dissolved oxygen concentration and to maintain stability of the plating solution, there have been proposed various methods for introducing an oxygen-containing gas into a plating solution and dissolving the oxygen in the plating solution (e.g. U.S. Pat. Nos. 4,152,467 and 4,632,852). But according to these U.S. patents, it is only described that air or oxygen is injected into the bath via conduits (U.S. Patent No. 4,152,467) and that an oxygen-containing gas is injected into the electroless copper plating solution (U.S. Pat. No. 4,632,852), and there is no description nor suggestion how to inject the oxygen-containing gas concretely. There was no problem so long as printed wiring boards have wiring pattern in larger size. But with recent requirements for higher density of printed wiring boards and finer wiring patterns, there arises a problem in that it is impossible to form wiring patterns uniformly and precisely by only injecting air or an oxygen-containing gas into the plating solution.
On the other hand, Japanese Patent Unexamined Publication No. 59-161895 discloses a plating apparatus wherein a gas dispersing pipe having a large number of small holes is installed at the bottom of a tank so as to supply oxygen-containing bubbles through the gas dispersing pipe. In this case, the diameter of the small holes is about 0.4 mm at least. Thus the diameter of the bubbles come out from these small holes becomes about 2 mm at least. The deeper the depth of the plating solution becomes, the larger the diameter of the bubbles at the plating solution surface becomes, although the diameter of bubbles may be small at the bottom of the tank. For example, when the depth of the plating solution is 1 to 2 meters, diameter of bubbles at the plating solution surface becomes several centimeters or more. When electroless plating of printed wiring boards, particularly those having fine wiring patterns is conducted under such circumstances, there arise problems in that the plating reaction is stopped at independent fine land portions, there takes place abnormal deposition on portions other than wiring patterns in the higher portions of plating wiring density, and the like. Further, there is also a problem in that copper is easily deposited on portions not directly contacted with bubbles from the gas dispersing pipe such as portions in the tank below the gas dispersing pipe, the bottom portion, hollow portions on the side walls of the tank, and the like. In addition, when the amount of oxygen-containing gas introduced is increased using a plating tank having the structure as mentioned above, since the diameter of bubbles is large, there arise problems in that the plating solution is vigorously agitated and a substrate to be plated is deformed by the bubbles, which results in causing contact with instruments and neighboring substrates and non-deposition or abnormal deposition on portions other than the pattern portions.